[8.16](backport #6724) [k8s] Correct agent image repository

[mergify]

What does this PR do?

This PR updates the registry of the elastic-agent container image to reflect the latest and correct one

Why is it important?

It is mandatory as elastic-agent container image has migrated to docker.elastic.co/elastic-agent/

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

None

How to test this PR locally

Follow either one of the elastic-agent Helm chart examples

Related issues

Closes #6725

---

This is an automatic backport of pull request #6724 done by Mergify.

[mergify]

Cherry-pick of ed7c19f has failed:

On branch mergify/bp/8.16/pr-6724
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit ed7c19f3b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   deploy/helm/elastic-agent/README.md
	modified:   deploy/helm/elastic-agent/values.yaml
	modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-hints/elastic-agent-standalone/kustomization.yaml.original
	modified:   deploy/kubernetes/elastic-agent-managed/elastic-agent-managed-daemonset.yaml
	modified:   deploy/kubernetes/elastic-agent-standalone/elastic-agent-standalone-daemonset.yaml
	modified:   dev-tools/packaging/packages.yml
	modified:   docs/local-k8s-testing.md
	modified:   pkg/testing/kubernetes/supported.go
	modified:   skaffold.yaml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   deploy/helm/edot-collector/kube-stack/values.yaml
	both modified:   deploy/helm/elastic-agent/examples/eck/rendered/manifest.yaml
	deleted by us:   deploy/helm/elastic-agent/examples/fleet-managed-ksm-sharding/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/fleet-managed/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/kubernetes-default/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/kubernetes-hints-autodiscover/rendered/manifest.yaml
	deleted by us:   deploy/helm/elastic-agent/examples/kubernetes-ksm-sharding/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/kubernetes-only-logs/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/multiple-integrations/rendered/manifest.yaml
	deleted by us:   deploy/helm/elastic-agent/examples/netflow-service/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/nginx-custom-integration/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/system-custom-auth-paths/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/user-cluster-role/rendered/manifest.yaml
	both modified:   deploy/helm/elastic-agent/examples/user-service-account/rendered/manifest.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/default/elastic-agent-managed/base/elastic-agent-managed-daemonset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/default/elastic-agent-standalone/base/elastic-agent-standalone-daemonset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-autosharding/elastic-agent-managed/base/elastic-agent-managed-daemonset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-autosharding/elastic-agent-managed/extra/elastic-agent-managed-statefulset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-autosharding/elastic-agent-standalone/base/elastic-agent-standalone-daemonset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-autosharding/elastic-agent-standalone/extra/elastic-agent-standalone-statefulset.yaml
	both modified:   deploy/kubernetes/elastic-agent-kustomize/ksm-hints/elastic-agent-standalone/kustomization.yaml
	both modified:   deploy/kubernetes/elastic-agent-managed-kubernetes.yaml
	both modified:   deploy/kubernetes/elastic-agent-standalone-kubernetes.yaml
	both modified:   internal/pkg/otel/README.md
	both modified:   internal/pkg/otel/templates/README.md.tmpl

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[pkoutsovasilis]

hmmm it seems that not all expected elastic-agent flavours of 8.16.4 exist under the new repository, thus I am gonna close this one